1. Field of the Invention
The present invention relates to a paper feeding method and apparatus, and more particularly to a method and apparatus for performing the feeding of a sheet of paper.
2. Description of the Prior Art
A paper feeding mechanism in a printer or the like is so designed that paper is fed by the length of predetermined minimum unit in correspondence with the one-step movement of a driving source such as pulse motor. Accordingly, in a case where the paper is to be fed by a certain length, the pulse motor or the like which is the driving source for paper feeding may be moved by the number of steps corresponding to the length.
However, the actual amount of feeding involves a relative error (it is slight, usually, 0.3-1%) on account of the slippage of the paper, caused by variation in friction, the expansion or contraction of the paper, the elastic deformation of a platen or roller made of rubber, etc.
For this reason, in a case where a large number of pages are to be printed in succession by using paper in which pages are continuous in the direction of paper feeding, such as fan-fold paper (paper called `continuous slip` in Japanese Industrial Standard JIS C6283), which is extensively used as paper for printers, the feeding errors are accumulated to gradually shift a printing position. Eventually, printing to be contained within a single page extends over two successive pages.
Moreover, in case of printing on sheets of paper each of which is printed with a prescribed form beforehand, the printing can protrude out of the form.
In order to prevent such drawbacks, mechanisms called `tractors` as shown in FIG. 18 and FIGS. 19A and 19B have heretofore been employed. These mechanisms are intended to maintain accurate paper feeding by bringing the protuberances (pins) 102 of the tractors into the feed holes 101 of sheets of paper. The mechanism of FIG. 18 uses belts 103 (tractor belts) having the protuberances, while the mechanism of FIGS. 19A and 19B uses cylinders (called `sprocket wheels`) having the protuberances 102 as indicated at numeral 201.
For fan-fold paper, the dimensions of the sheet of paper, the positions of the holes, etc. are stipulated by Japanese Industrial Standard (JIS) C6283 in order to ensure interchangeability.
There are many varieties of such tractors; they vary in shape and in the location in which they are installed, and some are permanently built in to printers, while others are removable.
Next, typical tractors will be briefly explained.
In the first place, the kinds of the typical tractors will be elucidated with reference to FIGS. 20 thru 23.
Throughout these figures, numeral 303 designates a sheet of paper, numeral 304 a platen, numeral 305 a direction of paper feeding, and numeral 306 a printing position.
In FIG. 20, the sheet of paper 303 enters a tractor 302 after passing through the printing position 306. The tractor 302 functions to draw up the sheet of paper 303. It is called a "front tractor".
In FIG. 21, the sheet of paper 303 enters a tractor 401 before passing through the printing position 306. The tractor 401 functions to push out the paper 303. It is called a "back tractor". Many of such tractors are built in printers by the use of sprocket wheels.
A tractor in FIG. 22 is called a "double-faced tractor". It utilizes both the front and rear surfaces of a tractor belt 501, and performs paper feeding drive at both positions preceding and succeeding the printing position 306.
A tractor in FIG. 23 is such that pins (sprocket pins) for feed hole drive are provided at both the edges of the platen 304. It is called a "pin feed platen".
Secondly, the problems of the respective tractors will be stated.